[RFC] Making talloc_reference() safer.

Andrew Bartlett abartlet at samba.org
Wed Oct 26 03:45:23 MDT 2011

On Mon, 2011-10-24 at 16:41 +1030, Rusty Russell wrote:
> Hi all,
>         git://git.samba.org/rusty/samba.git #talloc-reference-check-wip
>         I had the fun of re-arguing talloc_reference() safety with
> Tridge last Thursday.  We agree that talloc_reference fills a real need,
> but we can make it safer by expanding the checks which differentiate
> normal from refcounted objects.
> Firstly, note that talloc_reference() has several real use cases:
> anywhere that reference counts would normally be used.  The patterns
> I've seen are:
> 1) A "cache" of objects, where the cache may evict objects.  The cache
>    wants to hold a reference, as do the callers, and the object is freed
>    if it's evicted from the cache *and* noone else has referenced it.
> 2) A "dealer" of single objects, such as the tdb_wrap code which will
>    keeps track of all tdbs and avoids reopening the same tdb by handing
>    back a referenced tdb.

Which of these do the python bindings fit into?

All samba talloc objects can be exposed into python, and in particular
NDR objects are used in python a lot.  The current talloc modal works
well here, as we take a talloc reference each time we expose the object
into python, and we take a talloc reference each time we hold an
python-originated object into a long-term C structure (such as
gensec_start needing credentials).  This ensures that the object, which
in C may be held by implicit rules of 'you would be mad to free *that*
before this finishes' isn't taken away by python's garbage collector.

The particular challenge is that any object we have a python binding for
needs this behaviour, and we expose a *lot* of objects to python (every
IDL struct, and many other interfaces besides).

I'm concerned that the discussion so far has not addressed this
important use case, yet Samba's python bindings have become an integral
part of the project, and cannot be ignored. 


Andrew Bartlett

Andrew Bartlett                                http://samba.org/~abartlet/
Authentication Developer, Samba Team           http://samba.org

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